function mod_symbols = tx_modulate(bits_in, modulation)

full_len = length(bits_in);

% BPSK modulation
if ~isempty(findstr(modulation, 'BPSK'))
   % BPSK調制：0->-1；1->1
   mod_symbols = bits_in*2-1;
% QPSK modulation
elseif ~isempty(findstr(modulation, 'QPSK'))
   % Angle [-3*pi/4 -pi/4 pi/4 3*pi/4] corresponds to 
   % Gray code vector [00 10 11 01], respectively.
   table=exp(j*[-3/4*pi 3/4*pi -1/4*pi 1/4*pi]);
   inp=reshape(bits_in,2,full_len/2);
   mod_symbols=table([2 1]*inp+1);  % maps transmitted bits into QPSK symbols
else
   error('Unimplemented modulation');
end